MAIDUGURI, Nigeria (AP) — Islamic extremists threatening a bloodbath forced thousands of people from villages along Nigeria’s northeast, where refugees said Saturday the fighters have regrouped following a monthlong military crackdown.

People who escaped the bush near Nigeria’s border with Cameroon fled to the Borno state capital of Maiduguri said militants from the Boko Haram terrorist network also have written letters warning government workers to resign their jobs or face death. Other villagers left for Cameroon.

“They warned government officials and civil servants in Bama to resign or else face death in the next seven days. We are all scared, this could be more deadly, so we ran for our dear lives,” said Abba Fannami who fled to Maidguri with six family members.

A police officer said Boko Haram fighters were ransacking homes in neighboring Gwoza district, forcing residents to hide in caves in the rocky hills.

Soldiers and police on bombing raids with jet fighters and helicopter gunships have dislodged the insurgents from camps in a game reserve. But the refugees confirmed reports that the fighters have regrouped in the mountains and rocky hills of Gwoza and Bama districts.

In recent days the extremists — whose name means “Western education is sacrilege” — have targeted schools, killing 16 high school students and two teachers in two attacks.

The militants also have attacked primary schools, burning down at least 50 in the past year, according to Borno state commissioner for primary education, Tijjani Abba Ali.

In a separate operation in Kano state, west of Borno, police said they rounded up 400 migrants on Saturday and are deporting those who do not have the necessary documents.

“This is a mop up exercise of illegal immigrants to complement what the Federal Government is doing in the states currently under the emergency rule,” Kano state controller Hamisu Maishanu told reporters.

By late Saturday morning, he said screening of those detained had revealed some 150 people from the neighboring country of Niger who did not have the right documents.

Kano state is not in a state of emergency. The emergency covers the northeastern states of Adamawa, Borno and Yobe, an area encompassing 155,000 square kilometers (60,000 square miles).

The emergency and a military and police crackdown since May 14 has failed to crush the extremists blamed for the killings of more than 1,600 people since 2010. Boko Haram and splinter groups want to create an Islamic state in Nigeria, Africa’s most populous nation and the continent’s biggest oil producer.